USGS Water Resources Investigation Report 00-4145

Prepared in cooperation with the Air Force Center for Environmental Excellence

By John W. Lane, Jr., Peter K. Joesten, and Jennifer G. Savoie

ABSTRACT

A pilot-scale study was conducted by the U.S. Army National
Guard (USANG) at the Massachusetts Military Reservation (MMR) on Cape Cod, Massachusetts,
to assess the use of a hydraulic-fracturing method to create vertical, permeable
walls of zero-valent iron to passively remediate ground water contaminated with
chlorinated solvents. The study was conducted near the source area of the Chemical
Spill-10 (CS-10) plume, a plume containing chlorinated solvents that underlies
the MMR. Ground-water contamination near the source area extends from about
24 m (meters) to 35 m below land surface. The USANG designed two reactive-iron
walls to be 12 m long and positioned 24 to 37 m below land surface to intersect
and remediate part of the CS-10 plume.

Because iron, as an electrical conductor, absorbs electromagnetic energy, the US Geological Survey used a cross-hole common-depth, radar scanning method to assess the continuity and
to estimate the lateral and vertical extent of the two reactive-iron walls. The cross-hole radar surveys were conducted in boreholes on opposite sides of the iron injection zones using electric-dipole antennas with
dominant center frequencies of 100 and 250 MHz. Significant decreases in the radar-pulse amplitudes observed in scans that traversed the injection zones were interpreted by comparing field data to results of
two-dimensional finite-difference time-domain numerical models and laboratory-scale physical models.

The numerical and physical models simulate a wall of perfectly conducting material embedded in saturated sand. Results from the numerical and physical models show that the amplitude
of the radar pulse transmitted across the edge of a conductive wall is about 43 percent of the amplitude of a radar pulse transmitted across background material. The amplitude of a radar pulse transmitted through a hole
in a conductive wall increases as the aperture of the hole increases. The modeling results indicate that holes with an aperture of less than 40 percent of the dominant wavelength of the radar pulse are not likely to be
detected.

Based on the results of the numerical and physical modeling, the decreases in radar-pulse amplitudes observed in scans traversing the injection zones are interpreted as electrically
conductive zones that outline the distribution of iron. The area interpreted as iron in the northern A-wall contains two zones -- an upper zone about 10 m wide, extending from about 25 to 31 m below land surface, and a
lower zone about 8 m wide, extending from 31.5 to 34.5 m below land surface. The area interpreted as iron in the southern B-wall is about 9 m wide, extending from about 27 to 34.5 m below land surface. No discrete holes
were interpreted in either the A- or B-wall zones.

The interpretation of the field data suggests that (1) the hydraulic-fracturing method introduced iron into the subsurface, but not in the dimensions originally proposed; (2) the iron within the treatment
zones is distributed in a generally continuous manner; and (3) excluding the discontinuity in the A-wall, holes within the iron treatment zone, if any, exist at scales smaller than about 10 cm, the resolution limit of
the radar antennas and acquisition geometry used for this study. The cross-hole radar method appears to have been an effective method for delineating the distribution of iron in the two walls; however, the veracity of
the results cannot be ascertained without excavation or drilling into the treatment zone.